1. Field of the Invention
This invention relates to the art of making photosensitive electrodes and, in particular, to the techniques utilized to vapor deposit films of photoemissive material in forming a photocathode.
2. Description of the Prior Art
It is known (see for example U.S. Pat. No. 2,770,561 to Sommer, issued Nov. 13, 1956) to form a photocathode by providing an electron-emitting surface, such as a film of antimony activated or sensitized with one or more alkali metals, on a substrate, such as glass. The antimony film, as well as the layers of alkali metals, are typically obtained by vapor deposition from a source of material suitably disposed near the inside faceplate of the tube. This source of material, in the case of antimony, is generally several beads of platinum-antimony alloy, comprising about 50% antimony and 50% platinum, by atomic volume in accordance with the phase diagram for this alloy. The beads are formed by attaching, as by melting in a hydrogen atmosphere, particles of platinum-antimony to a platinum-clad molybdenum or tungsten wire.
The platinum-antimony alloy is generally prepared in a large billet under very controlled cleanliness conditions. This billet is then crushed into particles for later attachment to the platinum-clad molybdenum wire. Because the alloy is crystalline, the breakage of the billet is along particular grain boundaries of the alloy and the resulting particles are very irregular in shape and size. Screening these particles to a specific size or weight to obtain a controlled amount of deposited antimony is extremely difficult and where high volume is concerned, virtually impossible. In addition, attaching these particles to the wire creates further problems. The particles are attached by heating the wire to a selected temperature and then touching the particles to the heated wire. Due to the irregular shapes, melting and alloying of the particles to the wire is non-uniform and sporadic, causing the particles to spread out along the wire. This results in excess alloying and embrittlement of the wire making it difficult to handle the wire for mounting in the tube.
Since it is very difficult in photocathode processing to control the uniformity of the antimony deposition or to know the precise amount evaporated because of the irregular size or shape of the particles used to form the beads, a determination of the antimony thickness is made in the art by measuring the light transmission through the film. This light transmission measurement is generally done in a manner disclosed by Polkosky in U.S. Pat. No. 2,676,282, issued on Apr. 20, 1954. Curves are available that relate the light transmission to the thickness of the antimony film. Generally, more antimony than needed is included in the beads and when the desired light transmission ratio is obtained, the evaporation of the bead is stopped, leaving unevaporated antimony in the beads. Significant discrepancies in tube performances have, however, indicated an uncertainty regarding the accuracy of the light transmission-antimony thickness curves. In various tests performed to evaluate these curves, it has been found that for a given light transmission, the antimony thickness values may differ almost by a factor of two, due in part to the nonuniformity of the antimony film deposition.
It is thus apparent that more uniform and accurate film deposition techniques for forming photosensitive surfaces are desirable.